Báo cáo khoa học: The signature amidase from Sulfolobus solfataricus belongs to the CX3C subgroup of enzymes cleaving both amides and nitriles Ser195 and Cys145 are predicted to be the active site nucleophiles pot

The signature amidase from Sulfolobus solfataricus belongs to the CX 3 C subgroup of enzymes cleaving both amides and nitriles Ser195 and Cys145 are predicted to be the active site nucleophiles Elisa Cilia 1,2 , Armando Fabbri 1 , Monica Uriani 1 , Giuseppe G. Scialdone 1 and Sergio Ammendola 1,2 1 Centre of Biotechnology-Bioprogress, Anagni, Italy 2 AMBIOTEC SAS SS7, Cisterna di Latina, Italy Amide hydrolase (AH) and nitrilase (Nit) are two superfamilies of enzymes cleaving C–N bonds of differ- ent substrates [1]. These proteins all share the typical a ⁄ b hydrolase fold and can be grouped on the basis of their catalytic site and preferred substrate [2,3]. The enzymes of the Nit superfamily have a catalytic site formed by a Glu–Cys–Lys triad and can hydrolyse nitriles (EC 3.5.1.1) or the carboxylic amide group (EC 3.5.1.4) [4]. The AH superfamily includes the signature amidases, originally identified by primary structure analysis [5]. The substrate specificities and biological functions of these enzymes vary widely, but it has been shown that they all have a similar catalytic mechanism mediated by a Ser–cisSer–Lys catalytic triad [6–8]. Recently, the gene coding for the signature amidase from Sulfolobus solfataricus (SsAH) has been expressed in Escherichia coli and characterized [9]. This is a wide spectrum AH, converting carboxylic amides to their corresponding organic acids. We report that the SsAH enzyme also possesses nitrilase activity, a property shared by the well-characterized signature amidase from Rhodococcus rhodochrous J1 (RhorhJ1) [10,11]. Both enzymes contain an additional CX 3 C motif. To understand the structural basis of this dual spe- cificity, we constructed random mutants of SsAH and screened them for their activity on amide, nitrile and ester substrates. Mutants showing increased activity on nitriles all carried the Lys96Arg substitution, an obser- vation confirmed by site-directed mutagenesis. The structural framework of the observed mutations was investigated by constructing a homology model of the enzyme. The model suggests that a putative addi- tional catalytic triad formed by Ser171–Cys145–Lys96, where position 145 corresponds to the second cysteine of the additional CX 3 C motif, exists in this enzyme. We suggest that this alternative active site is respon- sible for the hydrolysis of nitriles, with Cys145 acting Keywords amidase; catalytic triad; CX 3 C; modelling; mutagenesis Correspondence S. Ammendola, Via Paduni 240-03012 Anagni FR, Italy Fax: +39 077 576 6541 Tel: +39 077 576 6824 E-mail: sergio.ammendola@fastwebnet.it (Received 14 June 2005, revised 18 July 2005, accepted 28 July 2005) doi:10.1111/j.1742-4658.2005.04887.x The signature amidase from the extremophile archeum Sulfolobus solfatari- cus is an enantioselective enzyme that cleaves S-amides. We report here that this enzyme also converts nitriles in the corresponding organic acid, simi- larly to the well characterized amidase from Rhodococcus rhodochrous J1. The archaeal and rhodococcal enzymes belong to the signature amidases and contain the typical serine-glycine rich motif. They work at different optimal temperature, share a high sequence similarity and both contain an additional CX 3 C motif. To explain their dual specificity, we built a 3D model of the structure of the S. solfataricus enzyme, which suggests that, in addition to the classical catalytic Ser-cisSer-Lys, a putative additional Cys-cisSer-Lys catalytic site, likely to be responsible for nitrile hydrolysis, is present in these proteins. The results of random and site-directed muta- genesis experiments, as well as inhibition studies support our hypothesis. Abbreviations AH, amide hydrolase; Nit, nitrilase; RhorhJ1, Rhodococcus rhodochrous J1; SH, serine hydrolase; Ss, Sulfolobus solfataricus; WT, wild type. 4716 FEBS Journal 272 (2005) 4716–4724 ª 2005 FEBS as the nucleophile involved into the thiol-mediated catalysis of nitriles. Sequence analysis suggests that other signature amidases, containing the additional CX 3 C motif, con- stitute a subgroup of the family and might all be able to cleave both amides and nitriles. Results Recently, the recombinant wild-type gene coding for the signature SsAH protein (SsAH-WT) has been cloned in E. coli and its product characterized. The enzyme exhibits high thermostability and thermophilic- ity [9]. The archeal enzyme converts several amide sub- strates in their corresponding organic acids producing stoichiometric amounts of NH 3 (data not shown). Consistent with previously published data, our prepar- ation of the purified recombinant SsAH-WT remains active at 70 °C for about 6 days and has a half-life of 5 h at 95 °C (data not shown). We found that SsAH-WT is able to convert both amides and nitriles, and in particular benzonitrile (Table 1). The specific activity of the enzyme on the Benzonitrile substrate is about 1 ⁄ 2000 with respect to its activity on benzamide (6 · 10 )4 lmolÆmin )1 Æ mg )1 and 1.18 lmolÆminÆ )1 mg )1 , respectively). This is similar to the observations for RhorhJ1, where the ratio between the two activities is about 1 ⁄ 6000 [11]. A sequence alignment of signature AHs, shown in Fig. 1, highlights blocks of structurally conserved resi- dues. These include the strictly conserved GSSXG motif. The first serine in this motif (Ser171 using SsAH numbering) forms the catalytic triad, together with Lys96 and Ser195 [12]. The second pattern, Cys141X 3 Cys145, is conserved only in a limited num- Table 1. Kinetic parameters of SsAH-WT and SsAH-K96R on different substrates. na, activity undetectable. Structure Substrate SsAH-WT SsAH-K96R K M (lmol) k cat s )1 K M (lmol) k cat s )1 Isobutyramide a 0.099 175 0.2 160 Propionamide a 0.102 181 0.093 160 Benzamide b 0.674 1192 0.723 1246 Cinnamamide b 0.434 769 0.526 906 Malononitrile b 0.0795 · 10 )3 0.140 0.609 · 10 )3 1.05 Tricloroacetonitrile a 0.5 · 10 )6 0.010 0.130 · 10 )3 0.225 Benzonitrile b 0.005 0.088 0.006 Phenylglycinenitrile 0.024 · 10 )3 0.043 9 · 10 )6 0.016 Cinnamonitrile a na na 9.607 · 10 )6 0.016 3-Anilinoproprionitrile a 0.157 · 10 )3 0.279 0.103 · 10 )3 0.177 a Assays were carried at pH 7.4 and T ¼ 70 °C using 61 lg purified protein. Activity was measured with the Berthelot assay, by reading the absorbance of the colour developed at k ¼ 630 nm. b Benzamide and benzonitrile values were determined by HPLC. E. Cilia et al. CX 3 C signature amidases can hydrolyze nitriles FEBS Journal 272 (2005) 4716–4724 ª 2005 FEBS 4717 ber of signature AHs, including RhorhJ1-AH and SsAH, both able to cleave nitriles (Fig. 2). We built a comparative model of SsAH using the program modeller with default parameters. In the model (Fig. 3) the Ser171–cisSer195–Lys96 triad is correctly positioned to perform catalysis with the Oc atom of Ser171 at about 2.8 A ˚ from the Oc atom of Ser195 and about 2.7 A ˚ from the amino group of Lys96. The analysis of the structural model reveals that Cys145, present both in SsAH and in RhorhJ1-AH, is in an ideal position to function as primary nucleophile in a second catalytic triad (Cys145–Ser171–Lys96) that could be responsible for the hydrolysis of nitriles. Indeed, the sulfur atom of Cys145 is at about 3 A ˚ from the Oc atom of Ser171 and 2.5 A ˚ from the amino group of Lys96 (Fig. 4). This putative alternative triad is located in a different plane from the active site Fig. 1. Multiple sequence alignment of signature AHs. The alignment was manually optimized taking into account the predicted or observed secondary structure also shown. Shaded regions indicate alpha helices (lighter) and beta strands (darker). The conserved GXSXG motif is underlined; identical residues (black triangles) represent the SsAH putative catalytic triad Ser171–cisSer195–Lys96. Black dots indicate the cysteine residues of the CX 3 C motif. RhorhJ1, Rhodococcus rhodochrous J1; PAM, peptide amide hydrolase; MaE2, malonamidase; FAAH, fatty acid amide hydrolase. CX 3 C signature amidases can hydrolyze nitriles E. Cilia et al. 4718 FEBS Journal 272 (2005) 4716–4724 ª 2005 FEBS catalytic triad and forms an angle of about 107° with its plane (Fig. 5A). Homology models can be useful to suggest functional hypothesis especially when they are based on a convin- cing evolutionary relationship, as in this case. Neverthe- less their conclusions need to be experimentally verified. In order to validate our hypothesis, we screened the activity of more than 40 SsAH mutants. The mutants were obtained by random, PCR-mediated mutagenesis and their activity assayed with the colorimetric method of Berthelot, appropriately modified for the assay con- ditions of the archaeal enzyme. This assay is based on the measurement of the colour that develops upon ammonia production in the reaction. Ammonia is pro- duced, in stoichiometrical amounts, upon substrate hydrolysis. Eight of the 40 mutants could hydrolyse most nit- riles more efficiently than SsAH-WT. The only amino acid substitution shared by all these mutants was the K96R mutation. To investigate the role of this muta- tion, we constructed the SsAH-K96R single mutant by site-directed mutagenesis. The 310-nucleotide fragment of the gene encoding SsAH was cut and used as tem- plate to introduce, by PCR, the mutation K96R. After amplification the fragment was purified and subcloned into pET3d vector and DNA sequenced for confirma- tion. The resulting plasmid was used to transform E. coli BL21(DE3) cells and the enzyme was expressed by isopropyl thio-b-d-galactoside (IPTG) induction as for the wild-type enzyme. Both SsAH-WT and SsAH- K96R were purified in a similar way and an identical protein concentration was used to compare their nitri- lase activity. The SsAH-K96R retains the ability to hydrolyse amides but hydrolyses nitriles more effi- ciently than the SsAH-WT (Table 1). The behaviour of this mutant can be rationally explained on the basis of the homology model, as the increased occupancy deriving from the arginine side chain in position 96 might modulate the size of the cleavable substrates (Fig. 4). Furthermore, the amino group of Arg96 could coordinate a water molecule that would be positioned sufficiently close to Cys145 to take part in the catalytic mechanism, thereby increas- ing the catalytic efficiency of the enzyme for nitrile substrates (Fig. 5B). The model suggests that this puta- tive additional water molecule cannot be accommoda- ted in the structure of SsAH-WT (Fig. 5C). Structural analysis of the model suggests that Arg96 may contrib- ute more effectively than Lys96 to the catalytic mech- NH2 COOH C145 S171 S195 K96 Fig. 3. The homology model of CX 3 C signature amidase from S. solfataricus. The side chains of the putative catalytic residues are shown. Ser171 and Lys96, shared by the two triads are in red. 141 145 171 P95896 PHMDATVVSRILDEAGEIVAKTTCEDLCFSGGSHTSYPWPVLNPRNPEYMAGGSSSGSAV 177 A AO55930 PSEDATVVKRLLAAGATVVGKSVCEDLCFSGASFTSASGAVKNPWDLARNAGGSSSGSAV 177 ZP_00124054 PSEDATVVKRLLAAGATVIGKSVCEDLCFSGASFTSATGAVKNPWDLTRNAGGSSSGSAV 177 BAC99079 PSRDATVVTRLLAAGATVAGKAVCEDLCFSGSSFTPASGPVRNPWDPQREAGGSSGGSAA 177 CAD36560 PSRDATVVTRLLAAGATVAGKAVCEDLCFSGSSFTPASGPVRNPWDRQREAGGSSGGSAA 177 S38270 PRYDATVVRRLLDAGATITGKAVCEDLCFSGASFTSHPQPVRNPWDESRITGGSSSGSGA 177 P27765 PGFDATVVTRLLDAGATILGKATCEHYCLSGGSHTSDPAPVHNPHRHGYASGGSSSGSAA 179 YP_046288 PEYDATIVTRMLDAGATILGKATCEHFCLSGGSHTSDPVAVHNPYRHGYSAGGSSSGSAA 177 NP_766838 PDFDATIVTRMLDAGAEIKGKVHCEHFCLSGGSHTGSFGPVHNPHKMGYSAGGSSSGSGV 177 ZP_00186529 PEFDATIVTRILDAGGEISGKAVCEHLCFSGGSHTSDTGPVLNPHDRTRSAGGSSSGSAA 177 * ***:* *:* : .* **. *:**.*.* .* ** :****.** Fig. 2. The CysX 3 Cys motif is conserved in a restricted group of signature amidases. Partial alignment of amidase sequences containing a second CX 3 C motif. From top to bottom (with database accession numbers in parentheses): Sulfolobus solfataricus (P95896); Pseudomonas syringae pv. tomato str. DC3000 (AAO55930); Pseudomonas syringae pv. syringae B728a (ZP_00124054); Rhodococcus globerulus (BAC99079); Rhodococcus erythropolis (CAD36560); is the Rhodococcus rhodochrous J1 (S38270); Pseudomonas chlororaphis (P27765); Aci- netobacter sp. ADP1 (YP_046288); Bradyrhizobium japonicum USDA 110 (NP_766838); Rubrobacter xylanophilus DSM 9941 (ZP_00186529). E. Cilia et al. CX 3 C signature amidases can hydrolyze nitriles FEBS Journal 272 (2005) 4716–4724 ª 2005 FEBS 4719 anism by activating Ser171 (Fig. 5B). One of its amino groups is about 2.6 A ˚ from the Oc atom of Ser171 and 2.5 A ˚ from the thiol group of Cys145. Even taking into account all of the limitations of molecular models of proteins with respect to experi- mental structures, we believe our model to be suffi- ciently reliable, being based on a respectable sequence similarity, to allow us to formulate the hypothesis that the triad Cys145–Ser171–Lys96 constitutes a second catalytic site. Our experimental analysis of the activity of the mutant SsAH-K96R on aliphatic amides and nitriles strengthens this hypothesis. Another line of evidence comes from the results of the inhibition studies shown in Fig. 6. Phenyl- methanesulfonyl fluoride inhibits amide conversion (more than 94%) and completely suppresses nitrile hydrolysis, supporting the hypothesis that the SsAH nitrilase activity exhibits a typical behaviour of sul- fhydryl enzymes. LYS96 sulso A B 1 M22B sulso 1 M22B GLY170 ASP191 CYS145 SER172 SER171 SER172 SER171 GLY170 GLY194 SER195 GLN192 GLY194 SER195 CYS145 ARG96 GLN192 ALA171 ALA171 THR223 GLU222 GLU222 THR223 ASP224 ASP224 ASP191 GLY193 GLY193 Fig. 4. Comparison between the putative catalytic sites of the SsAH-WT and SsAH-K96R models with those of the peptide amide hydrolase 1M22 template. The figure shows (in blue) the putative catalytic pocket of the SsAH-WT (A) and SsAH-K96R model (B) superimposed to the catalytic pocket of chain B of the template protein (orange). Both models show the Cys145Ala, Asp191Gln and Gln192Thr substitutions observed in the CX 3 C subgroup (SsAH numbering). SER195 SER171 SER195 SER171 CYS145 LY S 9 6 AB CD SER171 LY S 9 6 2.65 2.92 2.49 4.05 2.84 2.92 2.55 2.74 4.35 2.84 ARG96 CYS145 SER195 CYS145 LY S 9 6 H2O H2O H2O H2O ARG96 CYS145 SER171 SER195 Fig. 5. Closer view of the Ser171–cisSer195–Lys96 (A) and Ser171–Cys145–Lys96 (B) triads. The putatively catalytic triads are located in two different planes forming an angle of about 107°. Putative water molecules and hydrogen bonds in the catalytic site of the SsAH-WT and SsAH-K96R models are shown in (C) and (D), respectively. CX 3 C signature amidases can hydrolyze nitriles E. Cilia et al. 4720 FEBS Journal 272 (2005) 4716–4724 ª 2005 FEBS Discussion The characterization of the SsAH enzyme described here strongly suggests that the Ser171–cisSer195–Lys96 catalytic triad, similarly to other members of this family, forms the active site of this enzyme. Here we also suggest that a second catalytic triad might exist in this enzyme (and in other members of the family) involving Cys145 as well as Ser171 and Lys96. Our hypothesis is based on several lines of evidence. A 3D model, built on the basis of a reliable sequence alignment, shows that the members of this second putative triad are in the correct relative orientation. In our model, the Oc atom of Ser171, the amino group of Lys96 and the thiol group of Cys145 are sufficiently close to each other to form a catalytic site (Fig. 5A). Second, we isolated eight mutants with a higher catalytic efficiency on nitriles than the wild-type enzyme, and they all share the Lys96Arg mutation. On the basis of our model, this substitution is indeed likely to positively affect the catalytic efficiency. The role of this residue in nitrile catalysis has been confirmed by the construction and characterization of the single Lys96Arg mutant. Third, inhibition by phenylmethanesulfonyl fluoride suggests that the nitrile hydrolysis activity of the enzyme is likely to be thiol-mediated. Our proposal of the presence of two different cata- lytic triads for amide and nitrile substrates is in agree- ment with the mechanism proposed for nitrilases [13]. These are thiol enzymes that attack the cyano carbon of nitriles (R–C¼N) to form a covalent thioimidate complex. Addition of one water molecule is accom- panied by release of ammonia and transformation of the planar thioimidate to a planar thiol acyl-enzyme through a tetrahedral intermediate. Addition of a sec- ond water molecule allows the acid product to leave and regenerate the enzyme. Pace and Brenner observed that the geometric constraints of this reaction suggest that nitrilase facilitates interaction with linear ( 180°) substrate, planar ( 120°) thiomidate and acyl-enzyme intermediates, and tetrahedral ( 109.5°) water-bonded intermediate. In contrast, serine and thiol protease and amidase are confined to interacting with planar sub- strates and tetrahedral intermediates [2]. Our comparative model suggests that the two cata- lytic triads, with two different nucleophiles, each for a different C–N bond, are positioned in two different planes. When the catalytic base Lys96 is replaced by the chemically similar, but larger, arginine residue, the mutant enzyme hydrolyses only linear substrates, but is able to cleave the triple C–N bond more efficiently than the wild-type enzyme. Our model is able to explain this behaviour, as the arginine can coordinate a second water molecule (Fig. 5D). Notably, the putative catalytic cysteine is conserved in RhorhJ1-AH, another member of the family known to cleave amide ⁄ nitrile substrates. It is likely that other, not yet characterized, members of the family, containing the Cys141X 3 Cys145 motif also have a sec- ond, partially overlapping catalytic site, and might share the same enzymatic properties. Experimental procedures Wild-type SsAH expression and purification SsAH-WT was expressed in E. coli BL21(DE3)STAR, as previously described [9], and purified by diafiltration. The crude extract was loaded on a Vantage 32 column packed with Q-sepharose F.F. (Pharmacia, Fairfield, CT, USA) at 30 mLÆmin )1 and equilibrated with 20 mm Tris ⁄ HCl pH 8.5. First, the column was washed with 5 vols Tris ⁄ HCl 20 mm pH 8.5 (buffer A) and subsequently with 2 vols buffer A +50 mm NaCl. The active fraction was eluted with buffer A + 0.25 m NaCl. The eluted fraction was concentrated in nitrogen to the desired volume with an ultrafiltration cell (Amicon-Millipore mod. 8400; Billerica, MA, USA) equipped with an YM30 membrane. Protein content was determined using the BCA Kit (Pierce, Rockford, IL, USA) or by absorbance at k ¼ 280 nm (assuming E ¼ 0.973). Construction and expression of SsAH-K96R mutant For the random PCR reaction, two oligonucleotide primers were synthesized. Forward and reverse primers correspond to the first 20 nucleotides at the 5¢- and 3¢- of the gene cod- ing sequence and include the NcoI and BamHI sites, at each end, respectively. The 100-lL PCR reaction contained 1 · reaction buffer, 1.5 mm MgCl 2 , 0.25 mm each dNTP, 0.25 lm each primer. The amplification was thermocycled 100 77.3 5.8 0 87.3 74.7 101 99 95 90 100 94 0 20 40 60 80 100 120 Benzamide Benzonitrile % of conversion AB AB+Benzaldheyde 10mM AB+E64 1mM AB+PMSF 10mM AB+DDT 1mM AB+o-Phenanthrolyne 1mM Fig. 6. Inhibition of SsAH by different compounds. AB, assay buffer. E. Cilia et al. CX 3 C signature amidases can hydrolyze nitriles FEBS Journal 272 (2005) 4716–4724 ª 2005 FEBS 4721 for 30 cycles at 95 °C for 30 s, 50 °C for 30 s, and 72 °C for 90 s with 5 lLofTaq polymerase (minicycler MJ research- Genenco MC009904, Bakersfield, CA, USA). After amplifi- cation, the DNA fragments were cloned into the pET3d plasmid, purchased from Invitrogen (Carlsbad, CA, USA), and used to transform E. coli DH5 a grown on Luria–Ber- tani (LB) containing 100 lgÆmL )1 ampicillin. Plasmid DNA was extracted and purified making use of QIAprep (Qiagen, Valencia, CA, USA). The DNA inserts were sequenced by the M-MEDICAL service (http://www.mmedical.it). The E. coli BL21(DE3)STAR cells (Stratagene, La Jolla, CA, USA) were transformed with plasmids carrying the mutant genes, in order to screen for nitrile, amide and ester hydrolysis. Each colony was grown in a 50-mL flask containing 20 mL LB medium containing 50 lgÆmL )1 ampicillin and 34 lgÆmL )1 chloramphenicol. After about 16 h, protein synthesis was induced by adding 0.4 mm IPTG for 2 h. Cells were centrifuged at 5629 g for 20 min and cell pellets disrupted (one cycle for 1 min at 40% amplitude) with a cell sonicator (SONICS vibracell TM ). The recombinant enzyme was purified as described for the wild-type and used at the same protein concentration. Site-directed mutagenesis The SsAH-Lys96Arg gene was obtained by amplifying a short fragment starting from the initial ATG with the oligonucleotide forward primer: NcoFOR_1, 5¢-CTCTCC ATGGGAATTAAGTTACCCACATTGGAGGA-3¢, carry- ing the sequence for the NcoI restriction site, and a second oligonucleotide reverse primer K96R_rev, 5¢-ATATGTAT ACCCGCTATCATCACATTGTCCCTAATGCAAATCC TTTTTCC-3¢, that introduces the AfiG substitution in position 287 of the SsAH gene. Bstz17I. PCR amplification was performed at 95 °C for 30 s, 58 °C for 30 s, and 72 °C for 30 s with 2.5 U of Taq polymerase, in 1 · reaction buf- fer, MgCl 2 1.5 mm for 24 cycles; dNTP were used each at final concentration of 0.2 mm and oligonucleotides at 0.25 lm, respectively. The amplified DNA fragment was purified from 1.2% agarose gel and, after enzyme digestion, cloned in the pET3d plasmid carrying the SsAH-encoding gene previously deleted of the corresponding DNA frag- ment. After ligation, the plasmid was used to transform in the E. coli DH5a cells plated on LB containing ampicillin. The plasmid DNA was extracted and sequenced for con- firmation. The plasmid was used to transform E. coli BL21(DE3)STAR grown on LB agar containing ampicillin and chloranphenicol. Gene expression was induced by add- ing 0.4 mm IPTG for 2 h. All enzymes were from New England Biolabs (Beverly, MA, USA) or Roche (Indianapolis, IN, USA). Taq poly- merase was from Fermentas (Vilnius, Lithuania) and Euro- Clone (Milan, Italy). Media was LB broth from DIFCO (Franklin Lakes, NJ, USA). Nitriles, amides and corres- ponding acids were purchased from ACROS (Milan, Italy) or Sigma Aldrich (St Louis, MO, USA). All other reagents were from Sigma Aldrich, Carlo Erba (Milan, Italy) or Mallinckrodt Baker (Phillipsburg, NJ, USA) and IPTG was from INALCO (Milan, Italy). BCA protein assay KIT and Quanty-cleave protease assay kit were from Pierce. HPLC Nitriles, amides and acids were analysed by HPLC (Millen- nium Chromatography Manager equipped with PDA 996, Waters, Milford, MA, USA). An aliquot of 20 lL from the enzyme reaction mix was loaded to the column at a tem- perature of 40 °C and the molecules eluted with 4.5 mm HClO 4 . Propionamide and its acid were separated with an Rspack KC-811 (MIXED MODE) coupled with Rspack KC-G (Shodex) 8 mm · 300 mm and monitored at 210 nm. Propionic acid was eluted at a flow rate of 1.5 mLÆ min )1 . Benzonitrile, benzamide and benzoic acid were followed by using the Xterra C18 (Waters) column equilibrated with buffer A (20 mm KH 2 PO 4 pH 3) and eluted with an isocratic step of 4 min 0% buffer B and 12 min 50% buffer B (methanol). All molecules were monitored at 230 nm or by Refractive Index (1 ⁄ 8 · 10 )5 RIU-T.C. ¼ 0.5). Enzyme assays Berthelot assay Conversion of amides and nitriles was followed by using the Berthelot method [14] optimized for thermophilic enzymes. A 20-mL culture of transformed BL21(DE3) cells were grown overnight and protein synthesis was\in- duced with 0.4 mm IPTG for 2 h. Cells were centrifuged at 6000 r.p.m. for 10 min and resuspended in an appro- priate volume of 50 mm Tris ⁄ HCl pH 7.2. The suspension was sonicated for 1 min at maximum amplitude and 300 lL of cell crude extract were assayed in standard conditions. SsAH-WT and SsAH-K96R were used at 60 lgÆmL )1 and the acid produced was quantified by the amount of NH 3 + produced by reading absorbance of the colour devel- oped at k ¼ 630 nm. HPLC assay Enzyme specific activities were assayed, unless otherwise specified, using 7 lg pure protein incubated for 10 min with 7mm substrate in assay buffer. One enzyme unit was defined as the amount of enzyme that catalysed the forma- tion of acid (lmolÆmin )1 )at85°C. Protein stability at 70 °C was followed for up to 1 week and at 95 °C for 8 h, respectively. Aliquots of the enzyme incubated at fixed tem- perature, were assayed at different times. CX 3 C signature amidases can hydrolyze nitriles E. Cilia et al. 4722 FEBS Journal 272 (2005) 4716–4724 ª 2005 FEBS Inhibition of amidase activity Sixty micrograms of the Ss-AH were incubated at 80 °Cin 25 mm phosphate buffer pH 7.4 with 100 lg of benzamide for 30 min and benzonitrile for 16 h. All reactions were car- ried out in the presence ⁄ absence of/ 10 mm phenyl- methanesulfonyl fluoride or 10 mm benzaldheyde. Structural analysis Primary structure The primary structure of SsAH was aligned to AHs for which crystallographic or biochemical data were available, using clustalw at the EBI (http://www.ebi.ac.uk/clustalw/ index.html). Sequences of the rat fatty acid amide hydrolase (1MT5*), peptide amidase (1M22*), malonoamidase E2 (1OCK*), R. rhodochrous J1 and R. erythropolis N-774 (P22984) and MP50 (Q8KRD8), Pseudomonas syringae (v tomato, Q883E3) and Agrobacterium tumefaciens (Q9AHE8) were retrieved from the SwissProt Data Base [15]. Secondary structure Several different methods for the prediction of secondary structures were used (nnpredict [16]; prof [17]; psipred [18]). The consensus secondary structure prediction of each protein was used to manually refine the multiple sequence alignment. 3D model The 3D structure was built using the modelling package modeller, using the templates available from the PDB database, and analysed using rasmol [20], vega [21] or swisspdbviewer [22]. Models were optimized and evaluated using esypred3d [23]. Multiple sequence alignments were obtained by combining, weighting and screening the results of several multiple alignment programs. 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Trends Biochem Sci 20, 374–376. 20 Pedretti A, Villa L & Vistoli G (2003) VEGA: a versa- tile program to convert, handle and visualize molecular structure on Windows-based PC. J Mol Graph 21, 47–49. 21 Peitsch MC (1996) ProMod and Swiss-Model: Internet- based tools for automated comparative protein model- ling. Biochem Soc Trans 24, 274–279. 22 Jones DT (1999) Protein secondary structure prediction based on position-specific scoring matrices. J Mol Biol 292, 195–202. 23 Lambert C, Leonard N, De Bolle X & Depiereux E (2002) ESyPred3D: prediction of proteins’ 3D structures. Bioinformatics 18, 1250–1256. 4724 FEBS Journal 272 (2005) 4716–4724 ª 2005 FEBS CX 3 C signature amidases can hydrolyze nitriles E. Cilia et al. . The signature amidase from Sulfolobus solfataricus belongs to the CX 3 C subgroup of enzymes cleaving both amides and nitriles Ser195 and Cys145 are predicted. 2.6 A ˚ from the Oc atom of Ser171 and 2.5 A ˚ from the thiol group of Cys145. Even taking into account all of the limitations of molecular models of proteins